The width of the other person's screen is not always known. Usually, when it is known, it's 80 columns. If their mail or news reader doesn't automatically wrap lines, they'll have difficulty reading the message. Some older terminals will even chop off the lines at 80 columns. If there is quoted text which wraps, it can become confusing after a while who said what.
Line lengths of 65 characters or so are better, because they allow for a few levels of quoting before hitting 80.